A known microwave amplifier circuit is described in U.S. Pat. No. 3,857,106, issued to H. Seidel on Dec. 24, 1974 and assigned to the assignee of the present application. In accordance with this known circuit, an input signal is divided into two components by means of a quadrature hybrid coupler. The first of such components is applied, via a circulator, to the input port of a transistor amplifying stage. The second of such components is applied, via a second circulator, to the output port of the amplifying stage. A four-port reactive circuit combines the scattered waves produced at the input and output ports of the amplifying stage and delivers an amplified output signal. The availability of low-noise microwave transistors, such as gallium arsenide field effect transistors (GaAs FETs), should render possible the realization of a low-noise low distortion amplifier. The use of a single GaAs FET amplifier common to several radio channels imposes reliability requirements of substantial importance. On the one hand, failure of the amplifying transistor would cause unacceptable loss of signal for several channels. On the other hand, in case of power supply failure, the amplifying transistor would exhibit a high transmission loss resulting in a total service outage when used as a common amplifier in multichannel radio. The known arrangement described in Seidel's patent does not and cannot meet these reliability requirements by simple and economical means. Moreover, duplicating the number of amplifying stages to solve the reliability problem leads to a bulky, complex and expensive amplifier circuit.
Another known microwave amplifier arrangement is described in U.S. Pat. No. 3,789,314 issued to H. R. Beurrier on Jan. 29, 1974 and assigned to applicant's assignee. In this known arrangement, a signal source is coupled to a matching output circuit by means of two parallel connected wavepaths. One of the wavepaths is a low-loss passive wavepath, while the other wavepath includes one or more active elements. This known arrangement is primarily concerned with conserving a portion of the input signal that previously was dissipated in matching termination impedances. This known arrangement does not comply with the two reliability requirements mentioned above, i.e., acceptable output signals in the presence of either a failure of the amplifying stage or the power supply. Furthermore, a suggested coupling of two amplifying stages in parallel results in disadvantages similar to the one discussed above in connection with the Seidel arrangement.